A three phase, three level rectifier converts three phase input voltage signals into two output DC voltages that employ a common connection typically called a neutral point. This type of rectifier is often used in relative higher line voltage applications and provides a salient advantage in that the reverse voltage across the rectifying switches is reduced to half of the total output voltage or bus voltage. A common challenge in this type of multilevel converter topology is the problem of a voltage imbalance that often arises between the two output DC voltages.
This voltage imbalance can occur due to the two output DC voltages having different load currents thereby causing one output DC voltage to be a greater portion of the bus voltage than the other. This condition can also be caused by switching delays due to control errors, offsets and sensing problems. Ideally, the neutral point should be zero volts. However, voltage imbalance causes the neutral point voltage to drift or float thereby exacerbating regulation and control problems.
A hardware balancing circuit is often used to force the neutral point to be maintained at approximately zero volts. The hardware balancing circuit typically uses at least two balancing switches and an inductor connected across the output DC voltages. The balancing switches must be rated to withstand the entire bus voltage, often making them relatively expensive to use as is their added control and bias circuitry. The balancing switches also waste power and therefore reduce overall efficiency. Additionally, if the inductor should saturate due to an unusual load condition, reliability of the rectifier can be affected.
Accordingly, what is needed in the art is a more effective way to balance the neutral point voltage.